<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
 <style>
     nav{
         height: 100px;
         background-color: red;
     }
     .biao{
         position: fixed;
         left: 50%;
         top: 50%;
         transform: translate(-50%,-50%);
         width: 200px;
         height: 170px;
         background-color: green;
         z-index: 5;
     }
     .biao>div{
        width: 200px;
         height: 190px;
         background-color: yellow;
     }
     .beijing{
         position: fixed;
         left: 0;
         top: 0;
         width: 100%;
         height: 100%;
         background-color: rgba(0,0,0,.3);
     }
 </style>
<body>
    <nav>菜单菜单</nav>
    <div class="biao">
        标题
        <div></div>
    </div>
    <div class="beijing"></div>
    <div style="margin-top:6000px;"></div>
</body>
</html>